1. Field of the Invention
An object of the present invention is a decoder to decode an encrypted sound, said sound being encrypted so that it cannot easily be decrypted or deciphered by listeners who, moreover, would not possess means to decipher it. These means are usually called decoders. A decoder finds application more particularly in the field of broadcasting where encrypted sound is used so that listening to subscriber broadcasting channels is reserved for subscribers. In the field of broadcasting, it more particularly concerns T.V. broadcasting.
2. Description of the Prior Art
Subscriber T.V. channels are known. These channels transmit radioelectric signals representing images that may be in either clear form or enciphered form. These images and sounds said to be in clear form when they can be received on any television set provided with at least an antenna and demodulation means. On the contrary, when the images and sounds are transmitted in enciphered form, only the owners of a decoder can demodulate the radioelectric signals transmitted, and then demodulate them successfully. The decoding in question is done normally after a high-frequency demodulation of the transmitted radioelectric signal.
For the depiction of the images, the encryption encoding concerns a variable delay in the appearance of the line video signal with respect to the standard line triggering synchronization of the television station. For the electrical signal representing the sound, the encryption concerns a modulation, usually of the SSB (single sideband) type, of a carrier by the sound to be heard. In other words, subscriber TV broadcasting channels transmit a sound which, once the high frequency (HF) demodulation has been done, contains or does not contain an additional amplitude modulation of the SSB type, depending on whether it is encrypted or not.
When the TV broadcasting channel transmits in clear form, for example when showing commercials, the transmissions are not encrypted. Under these conditions, everybody can receive and understand the commercial. On the contrary, when a leisure-time program or a news program is transmitted, the radioelectric signal is encrypted. Whereas viewers without decoders find that their image gets scrambled at the same time as the sound becomes inaudible, this change occurs without any problems for those who possess a decoder. In other words, the decoder is capable of recognizing the presence of the encryption and of carrying out its deciphering function.
As the additional modulation of the sound is simple in character, subscriber networks seek to complicate the task of fraudulent persons who would like to receive the sound clearly by making it undergo only an amplitude demodulation after the high frequency demodulation. These subscriber networks then do the encoding by using a signal with a carrier of unknown frequency. Moreover, this unknown frequency may vary in time during one and the same transmission. Thus, fraudulent persons cannot decode the sound by means of a simple amplitude demodulator.
However, this additional encoding by modulation with an unknown frequency of the sound must have a simple decoding by the decoders provided by the subscriber T.V. broadcasting channel. There is a known system with which these decoders are provided in order to make the sound audible to owners of the decoder, whatever may be the state of encryption. This system essentially has a microprocessor that performs a computation, on the received signal, of the frequency of the carrier. This microprocessor then controls an oscillator frequency so that the oscillator emits a reconstituted carrier signal where the frequency of the carrier is equal to that of the unknown carrier of the encrypted sound. The drawback of such a system is that it requires the presence of a microprocessor and that a microprocessor such as this, although its use is becoming widespread and although it costs little in itself, increases the cost of the decoder. It is therefore desire to make a decoder that costs less while at the same time being functional to an equally high degree. In effect, since a microprocessor can be programmed, it accepts a certain programmability of the demodulation parameters.
Moreover, and because a phase of demodulation by a single signal, even if the frequency of this signal is unknown, does not sufficiently dissuade fraudulent persons with ingenuity, it has become the practice to make sound with single sideband modulation undergo a second additional modulation. This second modulation is also a single sideband modulation but is at another carrier frequency. This other carrier frequency, for its part, is a fixed frequency so as not to excessively complicate matters.
The result is that, ultimately, the sound may be modulated three times; once, the sound modulates the fixed frequency carrier signal, at a second time the result of this first modulation modulates the carrier with a frequency said to be unknown and, at a third time, the signal coming from this second modulation achieves a frequency modulation of an HF carrier so that it can be transmitted radioelectrically. This justifies the presence of the microprocessor which should be capable of telling the difference not between two situations, clear transmission and encrypted transmission, but three situations, clear, simply encrypted and doubly encrypted transmission. Although such a microprocessor appears then to be almost indispensable, the invention succeeds in doing without it.
In the invention, to overcome the drawbacks referred to, it is proposed to achieve decoding and demodulation of the signal modulating the carrier of unknown frequency in a demodulator that receives, firstly, the encrypted signal, i.e. modulating this unknown frequency and, secondly, a signal emitted by a voltage-controlled oscillator. The signal emitted by the oscillator results from a phase lock loop in which a phase comparison is made between the encrypted signal (received carrier) and a signal corresponding to the (reference) demodulation signal in this demodulator. When the two phases are identical (i.e. when the phase and the frequency of the reference signal are equal to the phase and to the frequency of the encrypted signal to be demodulated), the oscillator is kept at its demodulation frequency, and it changes it only when the modulation frequency of the encrypted signal itself changes.